As the James Webb Space Telescope (JWST) examines small and bright galaxies in the early universe, it may shed light on dark matter – the universe’s most mysterious matter.
That is the conclusion of scientists at the University of California, who conducted a simulation of the cosmos that tracks the formation of small galaxies – soon after the Big Bang. This seems to have raised the bar for the JWST.
Small galaxies, also called dwarf galaxies, are scattered across the cosmos, and scientists have suggested that they may be among the earliest formed galaxies. This means that dwarf galaxies are often considered crucial in studying the origin and evolution of the universe.
The problem, however, is that these galaxies don’t always match what astronomers expect to observe. For example, some run faster than expected, and others are less compact than simulations suggest. This is where dark matter comes into the picture.
These puzzling contradictions, according to scientists, could exist because researchers did not take into account the combination of gas and dark matter in their simulations.
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So the team’s new simulation took into account the interactions between dark matter and gas, and found that early galaxies were made smaller and much brighter than those in simulations that neglect the interaction. The scientists also saw that the galaxies were growing faster than other teams have seen.
That’s why the UCLA team believes astronomers should start hunting for small, early galaxies that are much brighter than expected using the JWST and other telescopes. If these galaxies don’t appear, then there could actually be something wrong with our dark matter theories.
In the dark about dark matter
Dark matter is such a headache for scientists because it doesn’t interact with light, making it essentially invisible to us.
The matter that makes up stars, gas, planets, our bodies, your neighbor’s cat and virtually everything you see around you consists of atoms made of electrons, protons and neutrons. These are called ‘baryons’ and interact with light. So scientists realized that dark matter must be made of something else: something ‘non-baryonic’.
All this means that despite the fact that dark matter makes up about 85% of the mass in the universe, scientists cannot detect it directly and have no good idea what it is made up of.
Because dark matter has mass, it interacts with gravity. This means that its presence can be inferred from the way these gravitational effects occur influence baryonic matter and indeed light.
The whole concept of dark matter was initially postulated because galaxies spin so fast that the gravitational influence of their baryonic matter alone could not prevent them from flying apart. It is the influence of invisible dark matter that ‘glues’ galaxies together through gravity, scientists believe.
Scientists further argue that most galaxies are surrounded by enormous halos of dark matter that extend far beyond their visible star, gas and dust content. They also think that these halos may have been integral to the formation and evolution of the galaxies.
In the current favored model of universal evolution, the ‘standard cosmological model’, the gravitational influence of dark matter clumps that existed in the universe 13 billion years ago managed to attract baryonic matter made of
normal old atoms.
Once this ‘ordinary matter’ became large enough, it collapsed and the first stars were formed. Along with dark matter, these first stars attracted more baryonic matter, creating the galaxies around them.
The Standard Model includes a form of dark matter called “cold dark matter,” which gets its name not because it is cold, but because it moves slower than the speed of light (heat is a measure of how fast particles move). The gathering of stars and galaxies in the standard cosmological model would also be slow if they depended on cold dark matter.
Baryonic matter in the form of hydrogen and helium gas from the Big Bang would have flowed past these slow-moving dark matter clumps at supersonic speeds at this early stage in the universe’s history. That is, until the matter eventually became entangled and then combined to form galaxies.
“In models that don’t take streaming into account, this is exactly what happens,” Claire Williams, a team member and doctoral candidate at UCLA, said in a statement. “Gas is attracted by the gravity of dark matter, forming clumps and knots dense enough for hydrogen fusion to occur, forming stars like our Sun.”
Williams and colleagues found that when this so-called streaming effect between dark and ordinary matter is taken into account in their simulation, part of the aptly named ‘Supersonic Project’ landed gas far from dark matter and growing galaxies. This prevented the immediate formation of stars.
Millions of years later, the gas eventually fell back into the galaxies, triggering an intense wave of star formation called a “starburst,” creating galaxies that had many more young, hot stars than regular small galaxies. For a time, these starburst galaxies should have seemed much brighter than other galaxies.
“Although streaming star formation was suppressed in the smallest galaxies, it also stimulated star formation in dwarf galaxies, causing them to outpace the non-streaming parts of the universe,” Williams explains. ‘We predict that the JWST telescope will be able to find regions in the universe where galaxies will be brighter, enhanced by this speed.
“The fact that they should be so bright could make it easier for the telescope to spot these small galaxies, which are typically extremely difficult to detect until 375 million years after the Big Bang.”
The fact that dark matter is effectively invisible means that these small, bright galaxies in the early universe would be a good benchmark to test the concept of cold dark matter. Failure to detect them could mean scientists will have to turn to other theories.
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‘The discovery of patches of small, bright galaxies in the early universe would confirm that we are on the right track with the cold dark matter model, because only the speed between two types of matter can produce the type of galaxy we are looking for. Smadar Naoz, Supersonic team leader and professor of physics and astronomy at UCLA, said in the statement. “If dark matter does not behave like standard cold dark matter and there is no streaming effect, these bright dwarf galaxies will are not found and we have to go back to the drawing board.’
The team’s research has been published in The Astrophysical Journal Letters,